A novel oxdiazole-containing polyfluorene copolymer (PF-OXD) was synthesized byattaching two electron-deficient, 4-
tert-butylphenyl-1,3,4-oxadiazole groups onto the C-9position of the alternating fluorene unit to form a 3-D cardo-structure. This polymer possessesa very high glass transition temperature (
Tg: 213
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C) and very good thermal stability dueto its rigid cardo-linkage. The results from the photoluminescence measurements of theisothermally heated PF-OXD thin film (150
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C for 20 h) show that the commonly observedaggregate/excimer formation in polyfluorenes is very effectively suppressed in this polymerdue to its 3-D structure and high
Tg. A double-layer LED device using this polymer as theemitting layer shows a bright blue emission with a low turn-on voltage at 5.3 V and a highbrightness of 2770 cd/m
2 at a drive voltage of 10.8 V. The maximum external quantumefficiency is 0.52% at 537 cd/m
2 with a bias of 7.4 V. The improved device performance overthat of poly(9,9-dioctylfluorene) (POF) may be due to better electron injection and transportin PF-OXD and the efficient energy transfer from the OXD side chain to the polyfluorenemain chains.